1966 M20 speedo gears

[Doug C.]

From what I can find in the archives it appears that there may be two different speedo drive gears to match up with certain driven gears (ratio dependent of course). Is this correct and if so what are the ratio ranges of the 2 drive gears, their part numbers, and differences (ie tooth count, tooth shape, etc.)
One message seemed to indicate that the drive gear for higher numerical ratios could be used for the lower ratios is this true. Thanks for any info.
p.s. my car has 3.36 gears with an L-79 327.

[Wayne M.]

Re: 1966 M20 speedo gears

Doug -- yes, 2 drive gears (on trans output shaft). Both 8 teeth, but different outer diameters [dia to fit 3.08, 3.36, 3.55 colored plastic driven gear ratios, and another to fit 3.70, 4.11, but not 4.56]. There is one driven silver gear for 3.70 ratios that will work on the drive gear for the 3.08 et al set. That's the only cross-over ratio.

Joe L. will probably respond to the details.

[Joe L.]

Re: 1966 M20 speedo gears

Doug-----

In addition to the information that Wayne provided, I'll offer the following:

First of all, speedometer gear "stuff" can get pretty complicated, so I'm going to provide information which has the widest applicability. This will cover MOST 1957-74 Corvette applications with 4 speed transmissions as well as 63-67 with Powerglide. The rear gear ratios covered will be 3.08 to 4.11. I'm not going to get into the numerically lower than 3.08 or numerically higher than 4.11. There weren't that many of those made and it gets too complicated for the purpose of this discussion.

For 1957 to 1967 applications with 4 speed and 63-67 applications with Powerglide there were 2 speedo DRIVE gears used. Both were steel gears which are an interference fit on the main shaft. These gears were GM #3708144 and GM #3708145 and both are of 8 tooth count. The GM #3708144 had an OD of 1.85". It was designed to work with nylon DRIVEN gears of 18 to 22 tooth (silver). These gears have an OD of 0.80" and are 0.35" wide. Generally, these combinations work with 3.08 to about 3.55 rear gears, depending upon tire size.

GM #3708145 had an OD of 1.77". It was designed to work with nylon DRIVEN gears of 22 tooth (green) to 25 tooth count. These gears have an OD of 0.87" and are 0.35" wide. Generally, these combinations work with about 3.70 to 4.11 rear gears, depending upon tire size.

1968-70 use 2 different DRIVE gears, too. These are GM #3924097 (later, GM #6261795) and GM #3924098 (later, GM #6261794). These DRIVE gears are nylon and are located to the main shaft with a special clip.

The GM #3924097/GM #6261795 are the same OD as the GM #3708145 DRIVE gear described above and work with similar rear gear ratios and the 22 tooth (green) through 25 tooth count DRIVEN gears.

The GM #3924098/GM #6261794 are the same OD as the GM #3708144 DRIVE gear described above and work with similar rear gear ratios and the 18 through 22 tooth (silver) count DRIVEN gears.

For 1971-74 just one DRIVE gear was used for all 4 speed applications. This was a steel gear of GM #3978758. It has an OD of 1.85", just like the GM #3708144, 3924098, and 6261794, and has an 8 tooth count. It works with the same 18-22 tooth (silver) DRIVEN gears as the aforementioned, too. For 71+, there was no DRIVE gear available to work with the 22 tooth (green) through 25 tooth count DRIVEN gears. For the numerically higher gear ratios of about 3.70:1 and higher, a speedometer adapter was used in conjunction with the 3978758 DRIVE gear and one of the 18-22 tooth (silver) DRIVEN gears.

[Verle R.]

Re: 1966 M20 speedo gears

Joe,

In your first main paragraph you state:

"I'm not going to get into the numerically lower than 3.08 or numerically higher than 4.11."

That implies that gears numerically lower than 3.08 were produced. What were they available in?

I well know about higher than 4.11; I tried 4.56, 4.88 and 5.12 in days of yore....

Verle

[Duke W.]

Re: 1966 M20 speedo gears

A 2.73 axle was available with some big blocks with Turbohydramatic, and the '75 L-48 with TH.

In '81 and '82 the two available ratios were 2.72 and 2.87:1.

Details are in the NCRS Spec Guide, which list the available drivetrain combinations.

Duke